Method for manufacturing multilayer ceramic capacitor

ABSTRACT

A method of manufacturing a multilayer ceramic capacitor includes the steps of: preparing a mixture of a raw material powder mainly composed of barium titanate particles; forming the mixture and a binder into a green sheet; alternately layering the green sheet and an internal electrode to obtain a laminated body; and sintering the laminated body. The step of preparing the mixture includes the steps of: introducing the raw material powder and the dispersion medium into a mixing container, and stirring them with balls serving as a mixing medium, to obtain a slurry containing a raw material powder mixture; and drying the slurry. The mixing medium has a diameter that is equal to or less than 400 times the mean particle size of the barium titanate particles of the raw material. The present invention provides a multilayer ceramic capacitor having good DC bias characteristics by suppressing the variation in crystal particles.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a multilayerceramic capacitor.

BACKGROUND ART

Recently, it has been required to reduce the size of multilayer ceramiccapacitors while enlarging their capacitance. To meet this requirement,the thickness of the dielectric layers must be reduced by using a rawmaterial powder with a small particle size.

The capacitance of multilayer ceramic capacitors is measured with an ACvoltage, not a DC voltage. However, a DC voltage is inevitably appliedin actual use, and this application of a DC voltage causes a decrease inthe capacitance of most capacitors. The decreased rate of thecapacitance caused by the application of a DC voltage is referred to asDC bias characteristics, and there is a demand for capacitors havinggood DC bias characteristics.

Prior art multilayer ceramic capacitors are manufactured as follows.

First, barium titanate powder, which is the main component, and powdersof metal oxides, which are sub-components and micro-additives, are mixedwith a dispersion medium of water and a mixing medium of zirconia balls,to form a slurry in which barium titanate and the metal oxides arehomogeneously dispersed.

Next, the slurry has the zirconia balls removed and is dried, to obtaina powder mixture.

Subsequently, this powder mixture is mixed with organic substances suchas a binder and a plasticizer, to produce ceramic green sheets.

Thereafter, the ceramic green sheets and internal electrodes arealternately layered, and the laminate is sintered. Lastly, externalelectrodes are formed, to obtain a multilayer ceramic capacitor.

As the prior art pertinent to this kind of multilayer ceramic capacitor,for example, Japanese Laid-Open Patent Publication No. 2002-226263 isknown.

Regarding the above-mentioned manufacturing method, this prior artproposes providing grain-growth-inhibiting particles at the grainboundary of crystal grains of barium titanate, to obtain good DC biascharacteristics. This proposal uses, for example, zirconia balls with adiameter of 3 mm as the mixing medium when barium-zirconium titanatepowder having a mean particle size of 0.3 μm is mixed with additives.However, since these zirconia balls are large in both diameter and mass,an excessive force is applied to the barium-zirconium titanate powderand the additives upon mixing, thereby resulting in excessive grinding.

If the barium-zirconium titanate powder or the barium titanate powder,which is the main component, is excessively ground, the particle size ofthe powder varies greatly. In addition, the excessively ground smallparticles facilitate grain growth during sintering, so that the particlesize of the resultant crystal particles becomes large.

When a DC voltage is applied to a multilayer ceramic capacitor includingsuch large crystal particles, a problem of deterioration of the DC biascharacteristics arises.

Accordingly, by suppressing the variation in crystal particles, thepresent invention aims to provide a multilayer ceramic capacitor havinggood DC bias characteristics.

DISCLOSURE OF INVENTION

A raw material powder mainly composed of barium titanate particles and adispersion medium are stirred with a mixing medium to form a slurrycontaining a raw material powder mixture, and the slurry is dried toobtain the raw material powder mixture. At this time, the presentinvention uses a mixing medium having a smaller diameter than theconventional one, thereby suppressing the excessive grinding of thebarium titanate due to the application of an excessive force. The methodof the present invention suppresses the variation in the particle sizeof the crystal particles, so that it is possible to obtain a multilayerceramic capacitor having good DC bias characteristics.

The present invention provides a method of manufacturing a multilayerceramic capacitor, comprising: the first step of introducing a rawmaterial powder mainly composed of barium titanate particles and adispersion medium into a mixing container, and stirring them with ballsserving as a mixing medium, to obtain a slurry containing a raw materialpowder mixture; the second step of drying the slurry, to obtain the rawmaterial powder mixture; the third step of forming the raw materialpowder mixture and a binder into a green sheet; the fourth step ofalternately layering the green sheet and an internal electrode, toobtain a laminated body; and the fifth step of sintering the laminatedbody. The mixing medium has a diameter that is equal to or less than 400times the mean particle size of the barium titanate particles before thefirst step.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a longitudinal sectional view of a mixing container forperforming a mixing step in Embodiments of the present invention.

FIG. 2 is a partially cut-away perspective view of a multilayer ceramiccapacitor 20 obtained in Embodiments of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention regulates the diameter of the mixing medium ofballs that are used for homogeneously mixing a raw material powdermainly composed of barium titanate particles, i.e., a raw materialpowder consisting of barium titanate particles and sub-componentparticles of metal oxides.

The present invention uses, as the mixing medium, balls having adiameter that is equal to or less than 400 times the particle size ofthe barium titanate particles of the raw material.

The particle size of the barium titanate particles of the raw materialis preferably 0.1 to 1.0 μm, and more preferably 0.1 to 0.5 μm. Thediameter of the mixing medium is preferably 200 μm or less. It is morepreferably 100 μm or less, and still more preferably 50 μm or less.Also, for the convenience of separating the mixing medium from the rawmaterial powder mixture after the mixing of the raw material powder, thediameter of the mixing medium is preferably at least 50 times theparticle size of the barium titanate particles of the raw material,specifically about 25 μm.

The amount of the dispersion medium is preferably 1 to 3 times thevolume of the raw material powder. By regulating the amount of thedispersion medium in the above range, it is possible to suppress theincrease in the variation in the crystal particle size of the resultantdielectric layers caused by the agglomeration of the raw materialpowder.

In the first step, the surface of the raw material powder is preferablycoated with the dispersion medium, i.e., moistened with the dispersionmedium, before being brought into contact with the mixing medium. Thiscan suppress the excessive grinding of barium titanate due to theapplication of an excessive force, and improve the dispersibility of theraw material powder.

In the first step, the temperature of the dispersion medium ispreferably 50° C. or less. This can suppress the change in energyapplied to the raw material powder.

In the first step, the amount of the mixing medium preferablyconstitutes 60 to 74% of the internal volume of the mixing container. Inthis case, the barium titanate particles and the sub-component particlesof metal oxides of the raw material powder can be effectively mixed witheach other.

In the second step, the drying temperature is preferably 120° C. orless. In this case, the agglomeration of the raw material powder can besuppressed, and the variation in the particle size of the crystalparticles can be reduced.

It is preferable that the second step further include the step ofdehydrating the slurry before the drying. In this case, theagglomeration of the raw material powder can be suppressed, and thevariation in the particle size of the crystal particles can be reduced.

It is preferable that the method further include, between the secondstep and the third step, the steps of: calcining the mixture; stirringthe calcined mixture, with a dispersion medium and a grinding mediumhaving a diameter that is equal to or less than 400 times the meanparticle size of the barium titanate of the raw material, to obtain aslurry containing a calcined powder; and drying the slurry, to obtainthe calcined powder. Accordingly, the main-component barium titanate andthe sub-components change moderately, so that it is possible to reducethe variation in the particle size of the crystal particles.

The grinding medium may have a diameter that is equal to or larger thanthe diameter of the mixing medium used in the first step, but thediameter is preferably 200 μm or less, in order to suppress theexcessive grinding of the calcined powder.

The calcined powder preferably has a specific surface area that is 0.5to 1 time the specific surface area of the barium titanate particles ofthe raw material. In this case, the dielectric layer after the sinteringis composed of crystal particles having a desired particle size.

The third step preferably includes the steps of: stirring the rawmaterial powder mixture, an organic binder and a solvent thereof, with athird mixing medium having a diameter that is equal to or less than 400times the mean particle size of the barium titanate of the raw material,to obtain a slurry; and forming a green sheet from the slurry. Bysuppressing the excessive grinding of the barium titanate due to theapplication of an excessive force, i.e., by suppressing the variation inthe particle size of the crystal particles, it is possible to obtain amultilayer ceramic capacitor having good DC bias characteristics.

The present invention is now described by way of embodiments.

EMBODIMENT 1

FIG. 1 is a longitudinal sectional view of a mixing container forperforming a mixing step in this embodiment. A mixing container 10includes a cylindrical container 11 and a cover 12 that seals the upperopening thereof, and is filled with zirconia balls 13, which are amixing medium. Reference character 14 is a stirring rod which isinserted in the mixing container through the cover 12 and has aplurality of stirring parts 15. The cover 12 has a sample inlet 16. Thecontainer 11 has a sample outlet 17 at its side face.

FIG. 2 is a partially cut-away perspective view of a multilayer ceramiccapacitor 20 obtained in this embodiment. Reference character 21represents a dielectric layer composed mainly of barium titanate, 22 and23 represent internal electrodes, and 24 and 25 represent externalelectrodes connected to the internal electrodes 22 and 23, respectively.

The manufacturing process of a multilayer ceramic capacitor is nowdescribed.

First, the starting material of the dielectric layer is weighed in thefollowing ratio. Per 100 mol of barium titanate, 1.0 mol of MgO, 0.3 molof Dy₂O₃, 0.3 mol of Ho₂O₃, 0.6 mol of SiO₂, and 0.05 mol of Mn₃O₄ areweighed as sub-components. Barium titanate particles having meanparticle sizes of 0.50 μm and 0.32 μm are used. Also, the mean particlesize of the sub-components is 0.1 μm or less, and there may be someparticles having a mean particle size of several nm.

Next, the starting material particles are mixed, water is added as adispersion medium, and the mixture is further mixed to coat the surfaceof the starting material particles with water.

If the amount of water is too small, the dispersibility of the rawmaterial powder deteriorates, and if it is excessive, the agglomerationof particles is likely to occur in the drying step. Therefore, theamount of the dispersion medium is desirably 1 to 3 times the volume ofthe starting material. In addition to the dispersion medium, adispersant capable of enhancing the dispersibility of the raw materialpowder may be added.

Next, the mixing step is described.

As illustrated in FIG. 1, the mixing container 10 has the rotatablestirring rod 14 therein, and is most closely packed with the zirconiaballs 13 having a mean particle size of 200 μm, i.e., in the state ofclosest packing. The zirconia balls 13 occupy about 70% of the internalvolume of the mixing container 10 excluding the volume of the stirringrod 14. The volume of the mixing container 10 is, for example, 0.5liter.

While the stirring rod 14 is rotated at a predetermined speed, forexample, a peripheral speed of 1 m/sec to 10 m/sec, the mixture of thewater and the raw material powder is introduced from the inlet 16 intothe mixing container 10 at a predetermined speed, for example, 0.1 to1.0 liter/min. The mixture of the water and the raw material powderpasses through the gaps between the zirconia balls 13 and flows out ofthe outlet 17. In this way, by passing through the mixing container, aslurry having excellent dispersibility can be obtained. The inlet 16 andthe outlet 17 of the mixing container 10 are provided with a filter (notshown), so that foreign matter does not enter the mixing container 10and only the slurry comes out of the outlet 17.

In the mixing container, the raw material powder including bariumtitanate collides with the zirconia balls and is ground a little.However, since the diameter of the zirconia balls is much smaller thanthat of the conventional ones, the excessive grinding of barium titanatedue to the application of an excessive impact can be suppressed.

In order to obtain the slurry with good productivity, it is desirable topack the mixing container 10 with spherical zirconia balls most closely.When packed most closely, the zirconia balls theoretically occupy 74% ofthe internal volume of the mixing container 10. If the zirconia ballsoccupy less than 60% of the internal volume of the mixing container 10,the powder cannot be mixed sufficiently, resulting in poordispersibility. Multilayer ceramic capacitors obtained from such rawmaterial powder has poor reliability.

Therefore, the zirconia balls 13 should occupy 60 to 74% of the internalvolume of the mixing container 10 (excluding the volume of the stirringrod), and preferably 70 to 74%.

Also, the rotating speed of the stirring rod 14 and the introducingspeed of the mixture are controlled such that an excessive force willnot apply to the barium titanate. For example, the peripheral speed ofthe stirring rod is set to about 6 m/sec, and the introducing speed ofthe mixture is set to about 0.3 to 0.5 liter/min.

Subsequently, the slurry is filtered for dehydration and dried in adrying room of which room temperature is 120%. When the dehydration isperformed before the drying, the agglomeration of the raw materialpowder during drying can be suppressed. When the drying is performed inthe drying room of the room temperature of 120% or less, theagglomeration due to rapid evaporation of moisture can be suppressed. At120° C. or less, the agglomeration of the raw material powder can besuppressed, but drying takes a long time if the temperature is low.Thus, the drying is preferably performed at 100 to 120° C.

Thereafter, the dried powder is calcined in air at 800 to 1,000° C. Forthe calcination temperature, optimum temperatures are selected dependingon the composition.

The calcination temperature and time are set such that the reactionbetween the barium titanate and the sub-components can be confirmed byX-ray diffraction of the obtained calcined powder. However, the specificsurface area of the powder after the calcination and grinding is set to0.5 to 1 time the specific surface area of the starting material ofbarium titanate, and the calcination temperature and time are controlledso as not to produce large amounts of agglomerated powder.

The calcination causes not only the above-mentioned reaction but also apartial reaction between the raw material particles. Thus, the reactedparticles are ground. With respect to the amount of dispersion medium ofthe water added in grinding, if it is too small, the dispersibility ofthe calcined powder deteriorates, and if it is excessive, theagglomeration of the powder is likely to occur in the drying step.Therefore, the amount of the dispersion medium added is desirably 1 to 3times the volume of the calcined powder. In addition to the dispersionmedium, a dispersant capable of enhancing the dispersibility of thecalcined powder may be added.

The grinding of the calcined powder is performed using the container 10as illustrated in FIG. 1. At this time, the calcined powder collideswith the zirconia balls 13 and is ground. However, since the diameter ofthe zirconia balls is much smaller than that of the conventional ones,the excessive grinding of the calcined powder due to the application ofan excessive impact can be suppressed.

The calcined powder is a powder which has reacted to some extent, sothere is a high possibility that it has a larger particle size than thatof the starting material. In order to grind/mix the calcined powderproperly, it is therefore desirable to use zirconia balls of which sizeis equal to or larger than that used for mixing the starting material,and preferably 200 μm or less.

Also, in order to obtain the slurry with good productivity, it isdesirable to pack the mixing container 10 with the zirconia balls 13most closely. When packed most closely, the zirconia balls theoreticallyoccupy 74% of the internal volume of the mixing container 10. If thezirconia balls occupy less than 60% of the internal volume of the mixingcontainer 10, the powder cannot be mixed sufficiently, resulting in poordispersibility. Multilayer ceramic capacitors obtained from such rawmaterial powder has poor reliability.

The zirconia balls used in this step should occupy 60 to 74% of theinternal volume of the mixing container 10, and preferably 70 to 74%.

The rotating speed of the stirring rod 14 and the introducing speed ofthe mixture are controlled such that an excessive force will not applyto the barium titanate.

Subsequently, the slurry is filtered for dehydration and dried in adrying room of which room temperature is 120° C. The dehydration beforethe drying can suppress the agglomeration during drying. The drying ispreferably performed at a room temperature of 120° C. or less in thesame manner as the first mixing step, and more preferably at 100 to 120°C.

By employing such drying temperatures, it is possible to effectivelyobtain dielectric layers composed of crystal particles having a smallvariation in particle size.

Next, using this dried calcined powder, a slurry for forming greensheets is prepared.

First, this calcined powder is mixed with alcohol, such as ethanol, tocover the surface of the calcined powder particles with the alcohol.

The calcined powder is then mixed with a solvent of n-butyl acetate, aplasticizer of benzyl butyl phthalate, and a binder of polyvinyl butyralresin, to obtain a slurry.

In this way, by coating the surface of the calcined powder particleswith alcohol before mixing it with the solvent, plasticizer, and binder,the agglomeration of the calcined powder particles can be suppressed.

If the amount of the alcohol used therein is excessive, desired ceramicgreen sheets cannot be obtained. Therefore, the amount of alcohol addedis set so as to be able to suppress the agglomeration of the calcinedpowder particles and coat the surface thereof. This amount is set toless than the total amount of the solvent, plasticizer, and binder.

Next, the above-mentioned slurry is applied onto an appropriatesubstrate, such as a sheet of polyethylene terephthalate, by the doctorblade process, to form a ceramic green sheet which serves as adielectric layer.

An internal electrode paste of Ni powder having a mean particle size ofabout 0.4 μm is screen-printed in a desired pattern on one face of theceramic green sheet.

Two kinds of ceramic green sheets, with internal electrodes of differentpatterns printed thereon, are alternately layered, heated, and pressedfor integration. In this case, the heating temperature is 80 to 140° C.,and the pressure is 100 to 200 kgf/cm². This sheet is cut into a size of2.4 mm wide and 1.3 mm long, to obtain a non-sintered laminated body.

The non-sintered laminated body is then put into a zirconia case withzirconia powder spread at its bottom, and heated to 350° C. in anatmosphere furnace with a current of nitrogen to burn the organicbinder. Subsequently, it is sintered in a current of a mixture gas ofnitrogen and hydrogen at 1,100 to 1,300° C., to obtain a sintered body.In the following example, it was sintered at 1,250° C. for 2 hours.

Thereafter, a copper paste is applied to end faces of the obtainedsintered body where the internal electrodes are exposed. The sinteredbody with the paste is baked in a nitrogen atmosphere in a mesh-typecontinuous belt furnace at 900° C., obtain a multilayer ceramiccapacitor as illustrated in FIG. 2.

By the above-described manufacturing steps, multilayer ceramiccapacitors were manufactured by using two kinds of barium titanatehaving mean particle sizes of 0.50 μm and 0.32 μm, and varying thediameter of the zirconia balls used for mixing the barium titanate ofeach particle size with the sub-components to 500 μm, 200 μm, 100 μm,and 50 μm. The DC bias characteristics of the multilayer ceramiccapacitors thus obtained were measured, and the results thereof areshown in Table 1. In grinding the calcined powder and preparing theslurry for forming the green sheets, zirconia balls having a diameter of500 μm were used.

TABLE 1 Mean particle Mean diameter DC bias Sample size of barium ofzirconia characteristics No. titanate (μm) balls (μm) (%) 1 0.50 500−53.8 2 0.50 200 −28.4 3 0.50 100 −23.3 4 0.50  50 −19.8 5 0.32 500−48.1 6 0.32 200 −32.5 7 0.32 100 −18.9 8 0.32  50 −14.8

DC bias characteristics were measured as follows. First, the multilayerceramic capacitor was retained at 150° C. for 1 hour, and then retainedat 20° C. for 24 hours. Subsequently, without applying a DC voltage, thecapacitance was measured. Thereafter, upon application of a DC voltageof 3.15 V to the same sample, the capacitance was measured. Thedecreased rate of this measured value relative to the value before theapplication of the DC voltage was used for DC bias characteristics.

According to Table 1, as in samples No. 2 to 4, 7, and 8, when thediameter of the zirconia balls is equal to or less than 400 times themean particle size of the raw material barium titanate, the DC biascharacteristics are equal to or less than −30%, which is good. That is,the dielectric layers thereof are composed of small crystal particleswith a small variation in particle size. Also, the smaller the diameterof the zirconia balls used for mixing, the better the DC biascharacteristics.

When the DC bias characteristics are greater than −30%, the designedcircuit constant varies significantly, so that desired circuitcharacteristics cannot be obtained. Therefore, this is not preferable interms of the use in actual circuits.

However, as in samples No. 1, 5, and 6, when the diameter of thezirconia balls is larger than 400 times the mean particle size of thebarium titanate, the DC bias characteristics are greater than −30%,which is not preferable in terms of the characteristics as thecapacitor. This is because the excessive grinding of the barium titanateduring mixing facilitates the grain growth of the dielectric layerswhile sintering, thereby resulting in an increase in the variation inthe particle size of the crystal particles.

Therefore, in order to obtain multilayer ceramic capacitors having goodDC bias characteristics, it is important that the particle size of thebarium titanate used as the starting material and its variation are asequivalent to the particle size of the raw material powder after the wetmixing and drying and its variation as possible.

Also, the smaller the mean particle size of the barium titanate, themore remarkable the effects. Therefore, as described in the aboveembodiment, it is effective to give special consideration to the size ofthe zirconia balls, in order to suppress the excessive grinding duringthe mixing of the raw material powder without impairing thedispersibility. Therefore, the zirconia balls used for the wet mixing ofthe powder, i.e., the mixing medium, should have a diameter that isequal to or less than 400 times the mean particle size of the startingmaterial barium titanate. Specifically, it is desirable to use smallballs having a diameter of 200 μm or less, preferably 100 μm or less,and more preferably 50 μm or less.

Next, multilayer ceramic capacitors were manufactured by setting thediameter of the zirconia balls used for the mixing to 50 μm and varyingthe size of the zirconia balls used for the grinding of the calcinedpowder. Their DC bias characteristics were measured, and the resultsthereof are shown in Table 2. Sample Nos. 4A to 4D use barium titanatepowder having a mean particle size of 0.50 μm, and sample Nos. 8A to 8Duse barium titanate powder having a mean particle size of 0.32 μm.

TABLE 2 Mean Specific particle surface Specific Diameter size of area ofsurface of DC bias barium barium area after zirconia character- Sampletitanate titanate calcination balls istics No. (μm) (m²/g) (m²/g) (μm)(%) 4A 0.50 3.2 2.8 500 −19.8 4B 0.50 3.2 2.8 200 −17.8 4C 0.50 3.2 2.8100 −16.5 4D 0.50 3.2 2.8 50 −16.3 8A 0.32 4.1 3.1 500 −14.8 8B 0.32 4.13.1 200 −13.1 8C 0.32 4.1 3.1 100 −12.4 8D 0.32 4.1 3.1 50 −12.2

The specific surface area of barium titanate or the ground ceramic rawmaterial was measured as follows.

First, the adsorption amount V_(m) (cm³/g), which is the amount of Headsorbed to the whole surface in the form of a monomolecular layer, isobtained from the following BET adsorption isotherm equation:x/{V(1−x)}=1/(V _(m) C)+x(C−1)/(V _(m) C)  (1)Specifically, three points are selected in a low relative-pressureregion of the actual adsorption isotherm of He in which x is plotted inabscissa and x/{V(1−x)} is plotted in ordinate, and a straight linepassing through these three points is obtained. At this time, the slopeof this straight line is expressed by (C−1)/(V_(m)C), and the interceptis expressed by 1/(V_(m)C). Thus, from the value of the slope of thestraight line and the value of the intercept, the adsorption amountV_(m) is calculated.

In the above equation (1), x is the relative pressure (adsorptionequilibrium pressure/saturated steam pressure), V is the adsorptionamount (cm³/g) of He at the relative pressure x, and C is a parametershowing the difference between the adsorption heat in the first layer ofHe and the adsorption heat in the second layer.

Next, from the monomolecular layer adsorption amount V_(m) obtained inthe above manner, the specific surface area S (m²/g) is obtained, usingthe following equation:S=sV _(m) K _(A) /V ₀  (2)In this equation, s is the cross sectional area (m²) of one He molecule,K_(A) is Avogadro's number, and V₀ is the volume of He per 1 mol (22,414cm³).

According to Table 2, when barium titanate having the same particle sizeis used as the raw material, the smaller the diameter of the zirconiaballs used for the grinding of the calcined powder, the better the DCbias characteristics. Such effect is particularly remarkable when thediameter of the zirconia balls is equal to or less than 400 times themean particle size of the raw material barium titanate, as in samplesNos. 4B to 4D, 8C, and 8D. It should also be noted that the smaller themean particle size of the barium titanate, the less the variationthereof, and the better the DC bias characteristics.

Therefore, when the grinding after the calcination is performed usingzirconia balls, it is desirable to use zirconia balls, i.e., a grindingmedia, having a diameter that is equal to or less than 400 times themean particle size of the starting material barium titanate. Preferably,it is desirable to use a medium having a diameter of 200 μm or less,more preferably 100 μm or less, and still more preferably 50 μm or less.

Also, when the specific surface area of the calcined powder is less than0.5 time that before the calcination, i.e., when the calcinatingtemperature is high, the powder is likely to agglomerate. Capacitorsmanufactured by using such material has poor DC bias characteristics.Therefore, it is desirable that the specific surface area of the powderafter the calcination be 0.5 to 1 time that before the calcination.

That is, in order to obtain multilayer ceramic capacitors havingexcellent DC bias characteristics, it is also effective to give specialconsideration to the diameter of the zirconia balls in grinding afterthe calcination, as well as in mixing the starting material.

Although zirconia balls were used as the mixing medium and the grindingmedium in this embodiment, any medium such as alumina balls may be usedunless it significantly changes the composition of the resultantdielectric layers.

Also, the heat treatment of the powder is performed in a short period oftime in such a manner that the heat history is as uniform as possible toavoid the agglomeration.

Further, although the raw material of the dielectric layers of thisembodiment included barium titanate as the main component and MgO,Dy₂O₃, Ho₂O₃ and the like as the sub-components, any raw material powdermainly composed of barium titanate can produce the above-mentionedeffects.

In preparing the slurry for forming the ceramic green sheets, the use ofthe mixing container as illustrated in FIG. 1 causes an increase in thetemperature of the dispersion medium of water in the stirring process.If the temperature becomes too high, it becomes difficult to obtain adesired slurry. Therefore, the temperature of the mixture of the waterand the ceramic powder is retained at 50° C. or less, and preferablybelow room temperatures. Also, although the mixing container having thestirring rod as illustrated in FIG. 1 was used in this embodiment, anycontainer may be used if it is capable of mixing the raw material powderwith the medium such as zirconia balls, and the stirring rod is notnecessarily needed.

In calcining the raw material powder, even if some water remains in theraw material powder, the water evaporates during the calcination, andhence, there is no adverse influence. In the case of not performing thecalcination, if the raw material powder after the drying has a largewater content, the powder is likely to agglomerate in the subsequentstep, which is not preferable. Therefore, in the case of not performingthe calcination, it is desirable that the weight of the raw materialpowder after the drying be equal to or less than 1.08 time, and morepreferably 1.05 time, the weight of the raw material powder upon theweighing.

In the above embodiment, the thickness of the dielectric layers was 3μm. However, as the result of examination of the thicknesses of about 1μm to 3 μm or less, it has been found that the DC bias characteristicsexhibit the same tendency depending on the diameter of the zirconiaballs used.

EMBODIMENT 2

First, in the same manner as in samples Nos. 4D and 8D of Embodiment 1,calcined powder is subjected to wet mixing, followed by drying.

Next, a slurry for forming ceramic green sheets is prepared by mixingthe dried calcined powder with a binder, etc. First, the dried calcinedpowder is mixed with alcohol, such as ethanol, to coat the surface ofthe calcined powder particles with the alcohol.

The calcined powder is then mixed with a solvent of n-butyl acetate, aplasticizer of benzyl butyl phthalate, and a binder of polyvinyl butyralresin.

In this way, by coating the surface of the calcined powder particleswith alcohol before mixing it with the solvent, plasticizer, and binder,the agglomeration of the calcined powder particles can be suppressed.However, if the amount of alcohol added is excessive, desired ceramicgreen sheets cannot be obtained. Therefore, the amount of alcohol addedis set so as to be able to suppress the agglomeration of the calcinedpowder particles and coat the surface thereof. This amount is set toless than the total amount of the binder, solvent, and plasticizer.

Thereafter, the above-mentioned slurry is passed through the mixingcontainer 10 as illustrated in FIG. 1, to obtain a slurry having gooddispersibility. At this time, the calcined powder also collides with thezirconia balls and is ground. However, since the diameter of thezirconia balls is much smaller than that of the conventional ones, theexcessive grinding of the calcined powder due to the application of anexcessive impact can be prevented.

There is a high possibility that the calcined powder used in the slurryfor forming the ceramic green sheets has a larger particle size thanthat of the starting material. Thus, in the same manner as in Embodiment1, it is also desirable in this embodiment to use zirconia balls ofwhich diameter is equal to or larger than that used for mixing thestarting material, in order to grind the calcined powder properly.However, the diameter is preferably 200 μm or less.

Further, in order to obtain the slurry with good productivity, it isdesirable to pack the mixing container 10 with spherical zirconia ballsmost closely. When packed most closely, the zirconia balls theoreticallyoccupy 74% of the internal volume of the mixing container 10. If thezirconia balls occupy less than 60% of the internal volume of the mixingcontainer 10, the powder cannot be mixed sufficiently, resulting in poordispersibility.

Therefore, the zirconia balls should occupy 60 to 74% of the internalvolume of the mixing container 10, and preferably 70 to 74%.

The rotating speed of the stirring rod 12 and the introducing speed ofthe mixture are controlled such that an excessive force will not applyto the calcined powder.

At this time, the calcined powder also collides with the zirconia ballsand is ground. However, since the diameter of the zirconia balls is muchsmaller than that of the conventional ones, the excessive grinding ofthe calcined powder due to the application of an excessive impact can besuppressed, so that it is possible to effectively obtain dielectriclayers composed of crystal particles having a small variation inparticle size.

The slurry obtained in the above manner is applied onto a substratesheet, such as a polyethylene terephthalate sheet, by the doctor bladeprocess, to form a ceramic green sheet serving as a dielectric layer.

Using ceramic green sheets thus obtained, multilayer ceramic capacitorsare obtained in the same manner as in Embodiment 1.

By this method, multilayer ceramic capacitors were manufactured byvarying the diameter of the zirconia balls used for the preparation ofthe slurry for forming the ceramic green sheets. The DC biascharacteristics thereof were measured in the same manner as inEmbodiment 1. The results thereof are shown in Table 3. Sample Nos. 4D-1through 4D-4 were manufactured under the same mixing andpost-calcination grinding conditions as those of Sample No. 4D. SampleNos. 8D-1 through 8D-4 were manufactured under the same mixing andpost-calcination grinding conditions as those of Sample No. 8D.

TABLE 3 Mean particle Mean diameter DC bias Sample size of barium ofzirconia characteristics No. titanate (μm) balls (μm) (%) 4D-1 0.50 500−19.8 4D-2 0.50 200 −17.5 4D-3 0.50 100 −16.8 4D-4 0.50  50 −16.3 8D-10.32 500 −14.8 8D-2 0.32 200 −12.2 8D-3 0.32 100 −11.8 8D-4 0.32  50−11.7

According to Table 3, when barium titanate having the same particle sizeis used, the smaller the diameter of the zirconia balls, the better theDC bias characteristics. Such effect is particularly remarkable when thediameter of the zirconia balls is equal to or less than 400 times themean particle size of the barium titanate, as in samples Nos. 4D-2 to4D-4, 8D-3, and 8D-4. Also, the smaller the mean particle size of thebarium titanate, the less the variation thereof, and the better the DCbias characteristics of the resultant multilayer ceramic capacitors.

Therefore, when the mixing is performed for preparing the slurry for theceramic sheets using zirconia balls, it is also desirable to usezirconia balls, i.e., a medium, having a diameter that is equal to orless than 400 times the mean particle size of the starting materialbarium titanate. Specifically, it is desirable to use a small mediumhaving a diameter of 200 μm or less, preferably 100 μm or less, and morepreferably 50 μm or less.

In order to obtain multilayer ceramic capacitors having good DC biascharacteristics, it is important that the particle size of the bariumtitanate used as the starting material and its variation are asequivalent to the particle size of the raw material powder for formingthe ceramic green sheets and its variation as possible. Therefore, it isalso effective to give special consideration to the diameter of thezirconia balls used in mixing the slurry for forming the ceramic greensheets after the calcination, as well as in mixing the startingmaterial.

Further, when a comparison is made among Table 1 with the varieddiameters of the balls used for mixing the raw material powder, Table 2with the varied diameters of the balls used for grinding the calcinedraw material, and Table 3 with the varied diameters of the balls usedfor preparing the slurry for forming the ceramic green sheets, it isfound that varying the balls used for mixing the raw material powder inTable 1 has the strongest effects on the DC bias characteristics. Thatis, from this embodiment, it is found that varying the diameter of theballs used for mixing the raw material powder is the most effectivemethod in terms of DC bias characteristics.

INDUSTRIAL APPLICABILITY

The present invention can provide a multilayer ceramic capacitor havingexcellent DC bias characteristics.

1. A method of manufacturing a multilayer ceramic capacitor, comprising:the first step of introducing a raw material powder mainly composed ofbarium titanate particles and a dispersion medium into a mixingcontainer, and stirring them with balls serving as a mixing medium, toobtain a slurry containing a raw material powder mixture; the secondstep of drying said slurry, to obtain said raw material powder mixture;the third step of forming said raw material powder mixture and a binderinto a green sheet; the fourth step of alternately layering said greensheet and an internal electrode, to obtain a laminated body; and thefifth step of sintering said laminated body, wherein said mixing mediumhas a diameter that is equal to or less than 400 times the mean particlesize of the barium titanate particles before the first step.
 2. Themethod of manufacturing a multilayer ceramic capacitor in accordancewith claim 1, wherein the mean particle size of said barium titanateparticles of the raw material powder is 0.1 to 1 μm, and the diameter ofsaid mixing medium is 200 μm or less.
 3. The method of manufacturing amultilayer ceramic capacitor in accordance with claim 1, wherein theamount of said dispersion medium is 1 to 3 times the volume of the rawmaterial powder.
 4. The method of manufacturing a multilayer ceramiccapacitor in accordance with claim 1, wherein the surface of the rawmaterial powder is coated with the dispersion medium before beingbrought into contact with the mixing medium in the first step.
 5. Themethod of manufacturing a multilayer ceramic capacitor in accordancewith claim 1, wherein the temperature of the dispersion medium is 50° C.or less in the first step.
 6. The method of manufacturing a multilayerceramic capacitor in accordance with claim 1, wherein the amount of saidmixing medium occupies 60 to 74% of the internal volume of the mixingcontainer in the first step.
 7. The method of manufacturing a multilayerceramic capacitor in accordance with claim 1, wherein the dryingtemperature is 120° C. or less in the second step.
 8. The method ofmanufacturing a multilayer ceramic capacitor in accordance with claim 1,wherein the second step further comprises the step of dehydrating theslurry before said drying.
 9. The method of manufacturing a multilayerceramic capacitor in accordance with claim 1, further comprising betweenthe second step and the third step the steps of: calcining said mixture;stirring the calcined mixture, with a dispersion medium and a grindingmedium having a diameter that is equal to or less than 400 times themean particle size of said barium titanate particles of the raw materialpowder, to obtain a slurry containing a calcined powder; and drying saidslurry, to obtain the calcined powder.
 10. The method of manufacturing amultilayer ceramic capacitor in accordance with claim 9, wherein saidgrinding medium has a diameter that is equal to or larger than thediameter of the mixing medium used in the first step.
 11. The method ofmanufacturing a multilayer ceramic capacitor in accordance with claim 9,wherein the specific surface area of said calcined powder is 0.5 to 1time the specific surface area of said barium titanate particles of theraw material powder.
 12. The method of manufacturing a multilayerceramic capacitor in accordance with claim 1, wherein the third stepcomprises the steps of: stirring said raw material powder mixture, anorganic binder and a solvent thereof, with a third mixing medium havinga diameter that is equal to or less than 400 times the mean particlesize of said barium titanate particles of the raw material powder, toobtain a slurry; and forming a green sheet from said slurry.